This application is based on Japanese Patent Application No.2000-184360, the content of which is hereby incorporated by reference.
The present invention relates to a projection system, and specifically relates to a projection system provided with a digital micro-mirror device (DMD) as an optical modulation element.
Digital micro-mirror devices (DMDs) have gained popularity as optical modulation elements for projection systems in recent years. DMDs have a surface of multiple micro-mirrors arranged in a matrix, and a single micro-mirror constitutes a single pixel of the surface image. The inclination of each micro-mirror is individually driven and controlled for optical modulation, and each micro-mirror has two inclination states corresponding to the ON state and the OFF state.
Illumination light is reflected toward the interior of the projection optical system by a micro-mirror in the ON state, and illumination light is reflected outwardly away from the projection optical system by a micro-mirror in the OFF state. Accordingly, only light reflected by the micro-mirror in the ON state arrives at the projection surface (e.g., screen) via the projection optical system, and as a result a display image comprising a shading pattern is formed on the projection surface.
FIGS. 13A and 13B are optical structure diagrams of a first conventional example of a projection optical system provided with a DMD. FIG. 13A shows the positional relationship between a DMD 2 and an image circle 3 via a projection optical system PL; and FIG. 13B shows the essential part of the projection system viewed from a perpendicular direction relative to the optical axis AX1 of the projection optical system PL. The projection optical system PL has a non-telecentric structure, and the illumination optical system IL comprises a D-cut lens and the like (other lenses are omitted from the illustration). AX2 represents the optical axis of the illumination optical system IL.
Light passing through the illumination optical system IL illuminates the DMD 2 from an inclination of 45xc2x0. Since the DMD 2 is constructed such that each micro-mirror has two inclination states (ON state and OFF state) centered on the axis ax, light is reflected toward the projection optical system PL by micro-mirrors in the ON state, and light is reflected outwardly away from the projection optical system PL by the micro-mirrors in the OFF state. As a result, the light reflected by micro-mirrors in the ON state forms a display image on the projection surface 1.
FIGS. 14 and 15 are optical structure diagrams showing the essential part of a second conventional example of a projection optical system provided with a DMD. FIG. 14 shows the optical path of projection light when the micro-mirrors of a DMD 2 are in the ON state, and FIG. 15 shows the optical path of projection light when the micro-mirrors of DMD 2 are in the OFF state. This projection optical system is provided with a total internal refraction (TIR) prism PR comprising a first prism PR1, and a second prism PR2. PL represents the projection optical system, and AX represents the optical axis of the projection optical system.
When the DMD 2 mirror rotation angle, i.e., the rotation angle of the micro-mirrors comprising the DMD 2, is xc2x110xc2x0, the use of a TIR prism allows an overall telecentric structure having a maximum F-number of 3.0. Accordingly, light use efficiency is increased, and a bright projection image can be obtained.
FIGS. 16 and 17 are optical structure diagrams showing the essential part of a third conventional example of a projection system provided with a DMD. FIG. 16 shows the optical path of the projection light when the micro-mirrors of the DMD 2 are in the ON state, and FIG. 17 shows the optical path of the projection light when the micro-mirrors of the DMD 2 are in the OFF state. The third conventional example is constructed so as to ensure that the back length of the projection optical system PL is longer than that of the second conventional example. For this reason, light that is reflected by the micro-mirrors in the OFF state to a position near the projection optical system PL avoids entering the projection optical system PL.
In the structure of the first conventional example, however, as shown in FIG. 13A, only an extreme part of the array of the DMD 2 can be used in the image circle 3. For this reason, in projection systems of the rear projection type requiring a wide angle of field, it is difficult to have an inexpensive projection optical system PL. Although it is necessary to use a D-cut lens and rear stop in the projection optical system PL in order to avoid interference between the projection optical system PL and the illumination optical system IL, when a D-cut lens is used in the illumination optical system IL, it becomes difficult to achieve uniform illumination distribution, as shown in FIG. 13B.
In a projection system of a one-chip type having a short distance from the DMD to the projection optical system PL, as in the case of the second conventional example, among the light reflected by the micro-mirrors in the OFF state, the reflected light near the projection optical system PL and the light reflected by the interior surface on the side surface of the second prism PR2 enters the interior of the projection optical system PL. This reflected light becomes ghost light and is the cause of contrast reduction. In the telecentric structure on the DMD 2 side, it is difficult to reduce magnification color aberration due to the high position at which the light rays enter relative to the positive lens on the telecentric side.
In the third conventional example shown in FIGS. 16 and 17, because the lens-back length pf the projection optical system PL is longer than the system of the second conventional example, the projection optical system PL is larger, more costly, and magnification color aberration also is increased. As shown in FIG. 17, the light reflected to a position near the projection optical system PL is emitted outwardly away from the projection optical system PL, but light reflected on the interior surface by the side surfaces of the projection optical system PL enters the projection optical system PL. This reflected light is the cause of contrast reduction.
In the video projector disclosed in Japanese Laid-Open Patent No. H8-251520, a construction is used to return the light reflected by the micro-mirrors in the OFF state to the light source in order to eliminate these problems. However, contrast reduction by the inter-surface ghosts of shared parts cannot be avoided because of the shared parts of the illumination optical system IL and the projection optical system PL.
In the projection optical systems used in digital television, adequate increase in contrast, compactness of the projection optical system, low cost, and adequate correction of magnification color aberration are demanded. Recently, in particular, as the so-called start of the era of digital broadcasting neared, there has been increasing demand for compact, inexpensive, and high-performance rear projection type digital television optical systems. Rear projection digital television devices must be entirely thin and compact. Furthermore, the projection image must have high contrast, high resolution, and low distortion.
Although thinness of the overall device can be achieved by using a wide field angle projection system, in general the number of lenses must be increased and the lens total length and the lens diameter must be increased in order to simultaneously achieve wide angle and high performance, such that increased cost cannot be avoided. These influences are particularly pronounced in an optical system requiring a long lens-back regardless of wide angle or short focal length. Moreover, these difficulties are markedly increased when telecentric characteristics are required on the DMD side.
When wide angle is required in a 1-chip DMD projection system, generally a large TIR prism and telecentric characteristics are required. Specifically, in a projection optical system having a screen angle of 2xcfx89=80xc2x0, the lens-back length must be approximately 2fxcx9c3f (where f is the focal length) by air conversion. This requirement inhibits the realization of a compact, inexpensive, high-performance digital video optical system of the rear projection type.
An object of the present invention is to provide an improved projection optical system.
In view of the previously described problems, an object of the present invention is to provide a projection system which is compact and capable of wide angle, and is a high-performance projection system producing a high contrast projection image.
These objects are attained by a projection system having the following construction
A projection system, in accordance with the present invention, directs light from a light source to a TIR prism unit, optically modulates all light reflected by the TIR prism unit via reflection by a DMD, and projects the modulated light passing through the TIR prism unit onto the projection surface of the projection optical system The projection system has a function of smoothing the light from a light source via the illumination optical system. The TIR prism unit has a first prism for completely reflecting light emitted from the illumination optical system, and a second prism for transmitting light passing through the first prism after optical modulation by the DMD. The projection system also satisfies the following condition:
100xe2x89xa6Xaxe2x89xa6250 
where Xa (mm) represents the distance along an optical axis of the projection system from the DMD to an exit pupil of the illumination optical system on a DMD side of the illumination optical system.